1. Field of the Invention
The present invention relates generally to an inverter power supply apparatus for metallic member joining or reflow soldering, and, more particularly, to an AC waveform inverter power supply apparatus allowing an AC waveform current to flow on its transformer secondary side.
2. Description of the Related Arts
Of late years, AC waveform inverter power supply apparatuses are prevailing as power supply apparatus for use in resistance welders. The AC waveform inverter power supply apparatuses can obviate the disadvantages of the DC inverter power supply apparatuses that flow a DC welding current on their welding transformer secondary side while keeping the advantages of the DC inverter power supply apparatuses.
More specifically, the DC inverter power supply apparatuses have a high heat generation efficiency enough to ensure stabilized resistance welding with less spatters since the length of their effective weld time is remarkably larger at all times than that of the current-unsupplied time, as compared with single-phase AC thyristor controlled power supply apparatuses. Due to their fixed current direction or polarities between a pair of electrodes clamping workpieces, however, the DC inverter power supply apparatuses may often suffer from such deficiencies that one of the pair of electrodes is liable to be degraded or wear than the other, that the workpieces tend to be magnetized, and that poor quality readily occurs because of the residual magnetism. Furthermore, high-frequency pulses from the inverter are converted into DC currents by a rectifying circuit on the secondary side of the welding transformer. Hence, the rectifying circuit and means to cool the same must be provided with a need for the high-frequency welding transformer suited to pass the AC pulses of the inverter frequency therethrough, which inconveniently makes the most prevalent commercial frequency welding transformers unavailable.
In this respect, the AC waveform inverter power supply apparatuses provide the same inverter control as the DC inverter power supply apparatuses, on their welding transformer primary side, to thereby advantageously achieve the heat generation efficiency and stability equivalent to the DC inverter power supply apparatuses, without the need for the rectifying circuit on the welding transformer secondary side, thus rendering low-frequency transformers available by flowing the AC waveform welding current (secondary current) at a low frequency substantially level with the single-phase AC power supply apparatuses.
Due to the availability of the low-frequency welding transformers in common with the single-phase power supply apparatuses as described above, the AC waveform inverter power supply apparatuses typically set the primary side AC waveform frequencies to the commercial frequency as shown in FIGS. 17A and 17B so that the weld time can be managed by the number of cycles of commercial frequency which has customarily been used in the single-phase AC power supply apparatuses. One cycle time being defined as one cycle 20 ms (or 16.6 ms) of the commercial frequency 50 Hz (or 60 Hz), the weld time Ta is managed by the numbers of cycles, e.g., four cycles [80 ms (or 66.4 ms)].
From experiences of the DC inverter power supply apparatuses, however, more and more users are recently hoping to arbitrarily set desired weld time by the time. Therefore, if the AC waveform inverter power supply apparatuses can set the weld time to any desired length by the time (e.g., ms), It may not merely be significantly convenient for the users but also it may be possible to further fine the weld time, i.e., one of welding condition parameter values and thus to achieve an Improved weld quality.
In the AC waveform Inverter power supply apparatuses, however, a difficulty may arise when the user desired weld time does not coincide with the integral multiples of the cycles of the welding transformer fundamental frequency. For example, when selecting the weld time to be 66 ms with respect to one cycle 20 ms of the fundamental frequency as shown in FIG. 18, 6 ms is left over from three cycles (60 ms). In this case, if during the remaining weld time (6 ms) the current supply Is made with one polarity only, then excessive residual magnetic flux will remain In the welding transformer, which may possibly break the inverter switching elements.